Retroviral vectors have been developed as an efficient system for delivering genes into many types of eukaryotic cells, in vitro. Success with in vivo gene transfer, however, has been limited by the lack of an effective delivery system. To address this problem, we are; 1. developing methods for introducing vectors or transduced hepatocytes/stem cells, directly into regenerating liver, initially in the rat animal model, and 2. attempting to create implantable bioartificial devices, which will function as substitutes for liver tissue, capable of expressing genes with therapeutic potential, indefinitely. The efficiency of gene transfer into the self-renewing hematopoietic/lymphoid stem cells is too low for retroviralmediated gene transfer to be applied successfully to human gene therapy. In an effort to improve the efficiency of gene transfer into stem cells, a three-dimensional culture system, based on a hollow-fiber/microcarrier perfusion bioreactor, is under development. In this system, bone marrow stromal cells or thymic epithelial cells will be cultured with the CD34+ marrow cell population, to high cell densities. These co-cultures will serve as "affinity columns" for pluripotent and T-lymphoid stem cells respectively.